1. Field of the Invention
The present invention relates to a power steering apparatus for controlling rotation of an electric motor using rotational speed based on vehicle speed and steering speed to assist steering with oil pressure generated by an oil pump which is driven by the electric motor.
2. Description of Related Art
Recently, a hydraulic power steering apparatus for supplying a power cylinder connected to a steering mechanism of a vehicle with hydraulic fluid from an oil pump which is driven by an electric motor to generate steering assistance force from the power cylinder has become widely used. Such a power steering apparatus is designed to determine indicated rotational speed, which is to be given to the electric motor, in accordance with a motor rotational speed map as shown in FIG. 4, which is defined by the steering speed and the vehicle speed, in order to obtain an optimal steering assistance force.
When short-time steering (rapid steering) at extremely high speed is applied to a conventional power steering apparatus constructed as described above, the indicated rotational speed to be given to the electric motor once reaches a peak value in accordance with the motor rotational speed map and then immediately decreases since the increase of the steering torque causes lowering of the steering speed. As a result, there is a problem that the flow rate of the hydraulic fluid runs short in such steering condition and the user feels an obstruction against his steering.
Although the shortage in the flow rate of the hydraulic fluid is solved by increasing the slope of rotational speed/steering speed so that high rotational speed is obtained even with low steering speed, there is a problem that the steering assistance force changes rapidly in such a method and the smoothness of the steering force deteriorates.
Described in Japanese Patent Application Laid-Open No. 2001-88725 is a control method of an electric power steering apparatus for reducing the feeling of an omission of steering force at low speed run to come close to the advantage of a hydraulic power steering apparatus.
Described in Japanese Patent Application Examined No. 2-40546 (1990) is an electric power steering apparatus for increasing a rotational output of an oil pump from a low speed base rotational line to a vehicle sensitive base rotational line, in response to steering when the vehicle is at a stop or at vehicle speed lower than or equal to preset low speed, or regardless of steering when the vehicle speed exceeds the preset low speed.
Described in Japanese Patent Application Laid-Open No. 3-32980 (1991) is a vehicle power steering system having a simple structure and shortening the response time extremely, wherein the system can be activated with little energy and exothermic of the electric motor can be decreased, by switching an electric motor off or into a standby mode in accordance with the vehicle speed.
Described in Japanese Patent Application Laid-Open No. 2000-142436 is a power steering apparatus by which constant steering feeling can be obtained even at the time of acceleration and deceleration.